Calculate the squared Euclidean distance between pairs of points and return a distance matrix

`distance(X1, X2 = NULL)`

X1

a `matrix`

or `data.frame`

containing real-valued
numbers

X2

an optional `matrix`

or `data.frame`

containing real-valued
numbers; must have `ncol(X2) = ncol(X1)`

The output is a `matrix`

, whose dimensions are described in the Details
section above

If `X2 = NULL`

distances between `X1`

and itself are
calculated, resulting in an `nrow(X1)`

-by-`nrow(X1)`

distance
matrix. Otherwise the result is `nrow(X1)`

-by-`nrow(X2)`

and
contains distances between `X1`

and `X2`

.

Calling `distance(X)`

is the same as `distance(X,X)`

# NOT RUN { x <- seq(-2, 2, length=11) X <- as.matrix(expand.grid(x, x)) ## predictive grid with NN=400 xx <- seq(-1.9, 1.9, length=20) XX <- as.matrix(expand.grid(xx, xx)) D <- distance(X) DD <- distance(X, XX) # }